Some Male Fish Incubate Eggs Fertilized By Others In Their Mouths


Stephen Luntz

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer

mouth and eggs
Image credit: StackCommerce

Certain species of fish go weeks with little to no food so they can hold eggs safely in their mouths. This remarkable act of parental care takes a twist with the discovery that some male fish do it when they aren’t even the biological parent of the eggs they’re doing so much to preserve. The reasons why this happens, like many details of the process, remain unknown.

Aquatic environments can be dangerous places for those unable to take evasive action. To prevent their eggs from becoming someone else’s dinner, many fish species have evolved “mouthbrooding”, where a parent holds the eggs in one place predators can’t access.


There are obvious disadvantages compared to giving birth to live young, since the presence of so many eggs makes it harder – if not impossible – for the brooder to feed. However, it does allow a fairer division of parental labor.

Charles Darwin University researchers decided to explore the process of mouthbrooding in two northern Australian fishes. The results proved sufficiently unexpected to be published in Biology Letters

First author, PhD student Janine Abecia, told IFLScience that like most mouthbrooding fish, it’s male Neoarius graeffei and Glossamia aprion charged with egg protection. “It seems to be a way the males can impress the females,” Abecia told IFLScience, winning more chances to mate. Take that, anyone who considers caring for children primarily the mother’s job.

Both fish live in extremely muddy rivers, making their behavior hard to study. The few observations we have come from aquariums, which may not reflect their actions in the wild. Mud doesn’t hide DNA, however, and Abecia and co-authors tested hundreds of members of each species to check if the eggs being cared for are actually the brooder’s offspring.


N.graeffei’s results were as expected – monogamous fathers cared for their young. G. aprion, which has the glorious common name "mouth almighty", was a different matter, often caring for the young of another father. Since it takes 4-5 weeks for mouth almighty eggs to hatch, that’s quite a sacrifice for the good of the species, or perhaps the ultimate in Piscean homoeroticism.

In some cases, the males were nursing eggs with a mixture of parents, including both theirs and another male. Other males were protecting eggs entirely fertilized by another. There were also cases where the male was carrying eggs he had fertilized, but which came from more than one female.

Abecia told IFLScience that the evolutionary basis for this is unknown. She didn’t check to see if the eggs belonged to a sibling, which might suggest a collective breeding strategy. Aquarium observations (and here the queasy may want to look away) report; “The male begins taking [the egg mass] in its buccal cavity when it is half-way out of the female,” the paper says. This doesn’t seem to leave a lot of time for fertilization – particularly by another male – and Abecia told IFLScience; “We don’t know when fertilization occurs.”

There have even been accounts of females chasing males and making them spit the precious eggs out, perhaps as a way of getting a demonstrably good partner to stop caring for another’s eggs and take her own instead.


Abecia added females also produce some “dummy eggs”, incapable of developing which they give male as a reward, prepping him for the long fast ahead.

As their names suggest, mouth almighties do indeed have enormous gob relative to their tiny size, although Abecia says this is as much to do with their status as ambush predators as their reproductive behavior.


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